The barrier property of a multilayer flexible packaging structure, in general, follows the following equation:P−1=P1−1+P2−1+P3−1+. . . Pn−1  (1)
Where P is total barrier of the multilayer flexible structure and P1 . . . Pn are barrier values for respective layer in the structure.
Ethylene vinyl alcohol (EVOH) has been one of the most effective barrier materials known to the flexible packaging industries, especially in providing excellent barrier for oxygen and aroma. However, the polar EVOH is not compatible with the non-polar polyolefinic film such as biaxially oriented polypropylene (BOPP). Thus, it requires an adhesion promoter or tie-layer resin such as anhydride-modified polyolefin in order to adhere to a non-polar polyolefinic substrate. Additionally, EVOH loses its barrier property if it is exposed to a high moisture environment. Therefore, it requires a moisture barrier layer for protection such as an anhydride-modified polyolefin or other non-polar layer or metallized layer.
U.S. Pat. No. 5,491,023 discloses a metallized film composition that is made in three steps: (1) coextruding a 3-layer composition of PP-tie/PP/sealant which is machine-direction oriented and flame treated, where PP-tie refers to a maleic anhydride modified propylene homopolymer or copolymer layer, (2) in-line extrusion coating with polyvinyl alcohol (PVOH) and further transverse-direction orienting and flame treating, and (3) vacuum metallizing onto the PVOH layer surface. The film composition can be described as: Aluminum/PVOH/maleic anhydride modified PP/PP/sealant. The oxygen barrier of the film was reported to be 0.37 cc/m2/day measured at 73° F. 0% RH.
U.S. Pat. No. 5,688,556 discloses a barrier film structure formed by vapor depositing a barrier coating on an coextruded film substrate such as (1) amorphous nylon or (2) EVOH/PP-tie/polymeric layer, of which the barrier layer of amorphous nylon or EVOH has a surface exposed for coating. The vapor depositing barrier coating could be an inorganic coating such as SiOx, Al2O3 and mixtures thereof, or an organic coating such as amorphous carbon coating. The barrier film composition can be described as vapor barrier coating/EVOH (or amorphous nylon)/PP-tie/PP (or PE, or biaxially oriented Nylon).
In the best example of U.S. Pat. No. 5,688,556, the EVOH layer was 0.06 mil (or 1.5 micron) thick, the oxygen barriers of above structure were 0.31 cc and 2.95 cc/m2/day depending on vapor deposition time of 22.5 or 11.25 seconds, respectively, for coated EVOH-OPP film. Without coating, 1.5 micron EVOH containing 48 mol. % of ethylene provided oxygen barrier of approximately 20 cc/m2/day at 73° F., 0% RH.
U.S. Pat. No. 5,827,615 discloses a metallized film substrate having a core layer and a metal receiving skin layer of ethylene vinyl alcohol (EVOH). The EVOH surface is metallized with aluminum. A low temperature sealable layer is coated to the aluminum surface. For the adhesion of EVOH, the core layer may either be blend with a maleic anhydride modified polyolefin or have an adhesion-promoting tie layer. According to the example, the film substrate of 2.13 mils (54.1 microns) containing 3 ga (0.75 micron) of EVOH layer, had oxygen barrier of 0.017 cc/100 in2/day (or 0.26 cc/m2/day) measured at 73° F., 0% RH. EVOH cited in this example 48 mol. % of ethylene, which is to that used in U.S. Pat. No. 5,688,556. This film substrate can be described as core layer/tie layer/EVOH/metal (aluminum)/sealant.
Moosheimer and H. Langowski in 42nd Annual Technical Conference Proceeding of 1999 Society of Vacuum Coaters describe a high barrier flexible structure. The structure consists of a vacuum-coated inorganic barrier layer such as Al, AlOx, or SiOx, and a high barrier lamination adhesive such as ORMOCER® to enhance the barrier property. ORMOCER® is a family of hybrid polymers made by sol-gel processing with inorganic and organic structural units to form network structure at temperature below 150° C., according to the text published in aforementioned conference proceeding by the leading author of K. H. Haas, et al. Four types of bifunctional monomers are used for synthesis of ORMOCER® polymers. The inorganic network is mainly based on Si—O—Si bonds using organically modified Si-alkoxices RxSi(OR)y, as starting compounds. Depending on the coating processes, organic network monomers may be a UV-crosslinking type or a thermal-crosslinking type. Typically hydrolysis and condensation reactions, by adding water and catalysts, occurred to form the crosslinking network structure that gives barrier and adhesion to film substrates. Typical ORMOCER® film thickness is in the range of 4 to 15 microns.
In short, the present state of the art of BOPP multilayer film applies EVOH coating alone on PP or with a metallized coating on the EVOH coating. Another commercial structure applies a sandwich structure EVOH/PE/Al on a BOPP layer where the Al layer is in contact with the BOPP layer, in which the EVOH is not directly contacting the Al layer. Neither of these structures provides an adequate oxygen barrier properties for long-term packaging of food products. There is a need in food packing applications where the oxygen barrier properties of the multilayer film should be substantially better than the barrier properties of either EVOH or aluminum metallized on an EVOH film.